In digging/loading work by a wheel loader, earth, sand or the like loaded in a bucket is often moved to a predetermined place. When traveling with the load, up and down movements of the vehicle cause a boom to swing up and down due to an increased weight of the bucket, and as a result, amplify vibrations of the vehicle.
With a view to suppressing these vibrations, a bottom pressure chamber of a boom cylinder and an accumulator are brought into communication with each other during traveling to have pressure fluctuations in the bottom pressure chamber absorbed in the accumulator, and during digging work with a bucket, on the other hand, the communication with the accumulator is cut off to prevent the absorption of digging force in the accumulator. As a conventional technique of this sort, reference can be made to JP-A-2000-309953.
FIG. 9 illustrates an essential part of the technique disclosed in JP-A-2000-309953 although it is not any of the drawings contained in this patent publication. Taking FIG. 9 as the conventional technique, a description will hereinafter be made. The conventional technique shown in FIG. 9 is provided with hydraulic cylinders arranged on a working vehicle, specifically boom cylinders 51, an actuator control valve for controlling a pressure in pressure chambers of these boom cylinders 51, specifically an actuator control valve 54, and main lines 52, 53 connecting the actuator control valve 54 and bottom pressure chambers 51a and rod pressure chambers 51b of the boom cylinders, respectively, and is also equipped with a branch circuit 50 branched out from the main lines 52,53. The branch circuit 50 has branch lines 55,56 branched out from the main lines 52,53, respectively, and these branch lines 55,56 are connected to an accumulator 58 and reservoir 61, respectively, via an opening control valve 57. Responsive to a signal from a controller 60, a solenoid valve 59 is switched so that the opening control valve 57 switched into a closed position G or open position H.
During digging work with the bucket, the opening control valve 57 is in the closed position G, so that the bottom pressure chambers 51a of the boom cylinder 51 and the accumulator 58 are kept out of communication to prevent the absorption of digging force, which is applied to the boom cylinders 51, in the accumulator 58.
During traveling, on the other hand, the solenoid valve 59 is switched to bring the opening control valve 57 into the closed position H. The bottom pressure chambers 51a of the boom cylinders 51 are, therefore, brought into communication with the accumulator 58 to have fluctuations in the load on the bottom pressure chambers 51a absorbed in the accumulator 58, so that vibrations of the vehicle can be suppressed.
In the above-described conventional technique, the opening control valve 57 is arranged to selectively bring the bottom pressures 51a of the boom cylinder 51 into communication with the accumulator 58 or to selectively cut off their communication. During the communication, the opening of the opening control valve 57 always remains constant. The weight of a front part, however, varies depending on the material or the like loaded on the bucket, the vehicle class, the front attachment, and the like. When the opening of the opening control valve 57 is excessively small relative to the weight of the front section, fluctuations in the load applied on a boom may not be fully absorbed in the accumulator 58 in some instances, in other words, vibrations may not be suppressed in some instances. When the opening is conversely too large relative to the weight of the front section, the vibrations of the front section may not be sufficiently attenuated so that the body of the working vehicle may be shaken to make it unstable in some instances. Accordingly, the conventional technique may not be able to fully absorb, in some instances, vibrations which take place as a result of a change in the weight of the front section.